Cell agglutination tests are performed routinely in the usual blood typing procedure for determining a patient's or donor's blood type according to the ABO classification system. In carrying out blood group identification tests, it is common to take a sample of blood from an individual and separate the blood into its component parts, i.e., red cells and serum. The cells are thoroughly washed in a saline solution and then placed, for example, in two cuvettes or other reaction vessels. One cell sample is mixed with a reagent of type A specific antibodies, otherwise known as type A antiserum or simply anti-A and the second with a reagent of anti-B. If the first cell sample agglutinates while the second does not, the individual's blood group is type A. If the reverse result obtains, then the individual's blood group is type B, while if agglutination takes place in both samples, the blood group is type AB. If agglutination is absent in both samples the blood group is type O.
The reason for blood typing is that the red blood cells of different individuals may have different antigens, or in other words, a different chemical structure on the surface of the cells. If, in transfusing blood to a patient, the donor's red blood cells had antigens different than those of the patient, the introduction of the donor's blood into the patient's blood stream would immediately stimulate the production of antibodies that would destroy the transfused blood cells and thereby vitiate the intended results of the transfusion. On the other hand, if the red cells of both a prospective donor and the patient have the same antigens, their bloods would be compatible and a safe transfusion could be effected.
It is so important to make a correct determination of an individual's blood type when a blood transfusion is contemplated or may become necessary that a double check, or reverse grouping, is ordinarily made in the typing procedure. Thus, in addition to observing if there is an agglutination reaction when the individual's red blood cells are mixed with antibody reagents, the individual's serum is mixed with standard red cells having known A antigens and B antigens. If the serum tests positively, i.e., agglutinates, with the A antigen cells and negatively with the B antigen cells, then the individual's blood group is type B. If the reverse result is observed, the blood group is type A, and if the results of both tests are negative, I.E., no agglutination, the blood group is type AB. If both tests are positive, the blood group is type O.
Another important test is the determination of whether or not a person's red blood cells have an Rh antigen. This antigen, like the others referred to, is important in determining compatibility of a donor's and recipient's blood when a transfusion is contemplated, but it is especially important in pregnancy where an Rh negative mother who developed Rh antibodies during a previous pregnancy may transfer those antibodies to a fetus having Rh positive blood and thereby effect the continuous destruction of the fetal blood cells. Prior knowledge of the patient's Rh condition enables preventive or compensatory measures to be taken. Other tests can be performed using other reagents to detect other red blood cell antigens should such further analysis be indicated.
The efficacy of the tests alluded to above is dependant on the ability of the technologist to observe the presence or absence of an agglutination reaction. Usually the initial determination is made by a direct unaided visual observation of the cell/serum-reagent mixture in the reaction vessel. A negative reaction, i.e., the absence of an agglutination product, is confirmed by studying the mixture under a microscope to assure that not even a weak agglutination occurred. Heretofore, the microscopic observation was made by pouring some of the cell/serum-reagent mixture onto a glass slide and observing the mixture under a microscope for some evidence of agglutination. It is obvious that a determination of a negative result when in fact there has been a weak agglutination result can lead to an erroneous conclusion as to the blood group type or at least to a discrepancy between the red cell group test and the reverse group test which would require re-performing the procedure to confirm or correct the initial results. In either event, the importance of accurately observing weak agglutination results is manifest.